1. Field of the Invention
The invention relates to a crosslinked acrylic heat-activatable adhesive with low activation temperature, and products containing said adhesive.
2. Related Art
Most embedded-lens and encapsulated-lens retroreflective sheetings, in particular for the Japanese traffic sign market, are heat applicable sheetings having a heat activatable adhesive. However, cube corner retroreflective sheetings are typically only adhered through the use of pressure-sensitive adhesives, which are substantially different from heat activatable adhesives. See, e.g., Adhesion and the Formulation of Adhesives 2d Ed., Wake, pp. 98-99 (Elsevier Applied Science Publishers 1986). Heat applicable cube corner retroreflective sheetings would be advantageous since they would have better handling characteristics, better quality, and easy maintenance.
A heat applicable cube corner retroreflective sheeting should have the desired optical clarity; have the desired level of tack, so that the sheeting is preferably positionable, but does not slide on the substrate once its position is fixed; have the ability to be applied at around 70.degree. C. or less by a heat lamp vacuum applicator ("HLVA") without loss of optical quality; adhere very strongly without any delamination or "pop off" failure; and be applicable to curved edge substrates such as the aluminum panels used for Japanese regulatory road signs.
Adhesives comprising nitrile rubber and an acrylic polymer, disclosed in Japanese Published Patent Kokai No. 88056274-B, are heat-activatable adhesives currently employed to adhere retroreflective sheetings to aluminum substrates in the Japanese traffic sign market. However, if this type of adhesive is applied to a cube corner retroreflective sheeting, the whiteness of the sheeting typically decreases because the color of the adhesive is dark brown and the sheeting is translucent. Further, when the heat-activatable adhesives of the prior art are laminated to a cube corner retroreflective sheeting wherein the sealing film of the sheeting has a surface treatment such as corona or chemical primer, the sheeting cannot hold on curved edge substrates because of their low cohesiveness at elevated temperatures. Because the heat activation temperature of the current heat-activatable adhesives is relatively high (about 82.degree. to 93.degree. C.), the brightness of the sheeting typically decreases due to thermal distortion of its retroreflective elements. Also, the nitrile rubber-based heat-activatable adhesives have little or no tack at room temperature. As a result, they are not appropriate for adhering cube corner retroreflective sheetings to aluminum because the sheeting shifts before HLVA processing.
A pressure applicable cube-corner retroreflective sheeting is not easily positionable because most pressure-sensitive adhesives have very high initial tack. Because the adhesive typically fails cohesively after application, the sheeting can only be applied to curved substrates that have a 127 millimeter ("mm") or greater radius. Such adhesives cannot hold on the curved edges of regulatory guidesigns in the Japanese traffic sign market which typically have a 7 to 8 mm radius. Further, sheetings applied using pressure-sensitive adhesives tend to trap air between the substrate and the sheeting during sign fabrication, so waste is typically high.
Similar requirements need to be met for the application of optically clear overlay films to traffic signs, Dinoc products and the lamination of light management films to glass or clear plastic surfaces. Overlay films typically must be laminated to their substrates without entrapment of air bubbles and without interfering with the optics of the product. Such films typically provide such unique properties as stain resistance, dew resistance and the like. Light management products, such as brightness enhancement films, light control films and privacy films for computer or monitor screens may be considered as special examples of overlay films wherein the primary function is control of the optical properties of the product.
Decorative films, such as for tile, tables and countertops, also make use of overlay films. In such decorative films, ease of application, including positionability and the ability to be bonded to the substrate without entrapment of air bubbles are of considerable importance.
Pressure-sensitive adhesives and tapes currently produced by ultraviolet ("UV") polymerization do not meet all the above requirements, especially with respect to tack, because the higher tack of known pressure-sensitive adhesives makes them very difficult to position. Known heat-activatable adhesives often lack the combination of optical clarity, high cohesive strength and low temperature of activation which is critical for lamination of microstructured surfaces such as those used in the retroreflective sheeting products of this invention.
Acrylic adhesives employing isobornyl acrylate are described in Japanese Published Patent Kokai Nos. 5(1993)-310810 and 6(1994)-128544, but these references do not teach or suggest their use in adhering retroreflective sheetings to substrates.
Acrylic adhesives employing N,N-dialkyl substituted amides are described in U.S. Pat. No. 4,946,742, U.S. Pat. No. 5,334,686 and EP 615 983 A2. While some of these disclose pressure-sensitive adhesives for PVC application, none disclose heat-activatable and optically clear adhesives used for the application of sheeting products.
A need exists for improved heat-activatable adhesives for adherence of the sheeting to desired substrates. A need also exists for heat-applicable sheeting products that may be used, for example, to enhance brightness, control light, help maintain the privacy of a computer screen, enhance the appearance of substrates, or create reflective areas on clothing or other articles to enhance the visibility of the user.